1. Field of the Invention
The present invention relates to contact lenses. Particularly, the invention pertains to monomers and dimers for making contact lens materials, and to contact lens materials and contact lenses made therefrom. Specifically, the monomers and dimers of the invention include an amide and a siloxane in their structures. The invention also relates to a process for making the above-described monomers, dimers, contact lens materials, and contact lenses.
2. Background of the Invention
Up until about January 1982, the contact lens market consisted of basically three types of contact lenses: rigid non-gas permeable lenses [for example, made of polymethylmethacrylate ("polyMMA")], gas-permeable rigid lenses (for example, made of silicone-containing monomers), and daily wear hydrogels (for example, made of poly-hydroxyethyl methacrylate). Since then, a new type of contact lens, an extended wear soft lens, has begun to penetrate the market and is having major impact on the future of the contact lens industry.
All of the extended wear contact lens materials that have been approved by the FDA to date can be characterized as high water content hydrogels. High water content hydrogels are not, however, viewed as being the "ultimate" polymer for the fabrication of extended wear contact lenses because of inherently weak physical properties. Also, the large polymer pore size of these high water content polymers usually leads to the formation of protein deposits that oftentimes penetrate into such lenses.
Hydrogel contact lenses tend to be weak, subject to deposit formation when worn in the eye, and lower in visual acuity than rigid contact lenses. All of these tendencies increase as the water content of the gels increase to the hydration levels required to obtain the degree of oxygen permeability needed for extended wear.
Rigid lenses, on the other hand, are less comfortable than hydrogel lenses. In a modern rigid lens, oxygen permeability is usually provided by siloxanes incorporated into the polymer. Siloxanes are usually hydrophobic by nature; siloxane-containing lenses have required wetting additives or surface coatings to provide adequate wettability for contact lens use.
Several approaches have been taken in the prior art toward solving the problem of wetting in oxygen permeable contact lenses made from siloxane polymers. Essentially all of the prior approaches have involved incorporation of a hydrophilic element in the polymer or copolymer system, to offset the hydrophobic nature of the siloxane.
Surface treatment, for example by generating free radicals through electrical discharge and subsequent attachment of polar radicals (including NH.sub.2) is proposed in the Gesser U.S. Pat. No. 3,925,178. Such surface treatments (i.e. replacement of surface hydrophobic atoms and groups with polar hydrophilic functional groups) have a tendency to be short-lived in their effectiveness due to normal wear and tear on the surface.
Copolymerization of a variety of hydrophobic siloxane monomers having a glycerolethyl functionality in the sidechain, with a hydrophilic monomer (such as N-vinyl pyrrolidone or dimethyl acrylamide), a methacrylic acid alkyl ester monomer, a cross-linking agent and an initiator, are described in a series of patents to Tanaka et al. (U.S. Pat. Nos. 4,139,513; 4,139,548; 4,139,692; and 4,235,985). The Tanaka et al. references do not teach or suggest the acrylamide sidechain siloxane monomers and copolymers of the present invention.
The copolymerization of hydrophilic amide group-containing monomers (eg. N,N-dimethyl acrylamide) with a polysiloxanylalkyl ester of acrylic or methacrylic acids is taught by Chang in U.S. Pat. Nos. 4,182,822 and 4,343,927. This does not teach or suggest the acrylamide sidechain siloxane monomers and copolymers of the present invention.
Difunctional acrylic siloxanes are at the heart of what is called the B&L (Bausch & Lomb) technology, exemplified by U.S. Pat. Nos. 4,153,641; 4,189,546 and 4,277,595 (to Deichert et al.); 4,254,248 (to Friends et al.); 4,259,467 and 4,260,725 (to Keogh et al.); and 4,276,402 (to Chromecek et al.). There, a variety of polysiloxanes, end-capped with polymerizable unsaturated groups, are shown to be useful for manufacturing contact lenses without the use of "fillers" such as cross-linking agents. The specifications of such patents suggest (but no specific teaching nor example is provided showing) incorporation of an acrylamido group adjacent each polymerizable unsaturated group of the difunctional monomers and specifically teach away from use of monofunctional monomers, as requiring such "fillers". Moreover, even in the disclosed difunctional embodiments (including the macromolecules), the tetrakis-(trisubstituted siloxy)disiloxane dimers of the present invention are not shown nor suggested.
Similarly, Mueller et al. (in the U.S. Pat. Nos. 4,136,250 and 4,277,582) describes difunctional macromolecules, for copolymerization with a water soluble monofunctional molecule [such as N-(hydroxymethyl) acrylamide.] Incorporation of an amide adjacent each polymerizable unsaturated group of the difunctional non-hydrophilic macromer is suggested, but no examples are specifically provided. Likewise, the tris-(tri-substituted siloxy)disiloxane dimer of the present invention is not shown nor suggested.
Morehouse, in U.S. Pat. No. 2,929,829 describes a process for the production of organosilicon acylamino compounds, including a proposed N-[tris(triethoxy)silylpropyl]acrylamide product, using gamma-aminopropyl triethoxysilane as a theoretical starting material. There is, however, no teaching nor suggestion of the tris-(tri-substituted siloxy)silylalkyl (meth)acrylamide monomers or copolymers of the present invention, nor is there any indication that such compounds would have any utility as contact lens materials.
Incorporation of an amide between an acrylate and a siloxane is shown in U.S. Pat. No. 3,249,461 (to TeGrotenhuls) as the reaction product of gamma aminopropyl trimethoxysilane and methacryloyl chloride. The suggested use for the product is as an adhesive; nowhere is usefulness as a contact lens material taught. The tris-(tri-substituted siloxy)silylalkyl (methy)acrylamides of the present invention are likewise not shown nor suggested.
It has been reported in the literature that siloxane containing polymers provide good oxygen permeability, but otherwise lack wetting properties and the ability to absorb water. [See Y. J. Shur, et al., J. Macromol Sci-Phys., B14, 565-572 (1977).] On the other hand, some polymers containing hydrophilic groups, such as poly(2-hydroxyethyl)methacrylate) ["poly(HEMA)"], poly(glyceryl methacrylate) ["poly(GMA)"] and poly(N,N-dimethylacrylamide) ["poly(DMA)" , can absorb water and have good wetting characteristics. The oxygen permeability of these water absorbable polymers depends upon the extent of hydration.
It is important to emphasize the significance of physical properties on lens performance. In order for a hydrogel to perform well as a contact lens, it must have enough "body", i.e. resilience, in the hydrated state to maintain a lens shape. At the same time, it must not be so rigid as to cause physiological problems. If the lens is too soft, it will drape and sag in such a way that it would be impossible to handle and also, the lens will deform too much in the eye, resulting in poor optics and poor visual acuity. If the lens is too rigid, it can cause physiological problems such as corneal staining, blanching, or even flattening of the cornea. Therefore, it is important to develop a develop a polymer with just the right resilience.
One factor directly affecting polymer resilience is the concentration of crosslinker. It is believed that a crosslinker, such as tetraethyleneglycol dimethacrylate ("TEGDMA"), randomly ties together long polymer chains, thus making the polymer network elastic. Polymer resilience is measured in terms of percent elongation. Based on polyHEMA, which has been accepted on the market for years and is used as a standard for contact lenses, a desirable percent elongation is 160-180%, while maintaining 2.5 to 3.0 g/mm.sup.2 tear strength.
The present invention demonstrates that a hydrogel/siloxane material will provide strong, highly oxygen-permeable contact lenses which are not prone to coating. Essentially this is a hydrogel which does not depend solely on water for its oxygen permeability. Thus, it is possible to maintain high oxygen permeability without having high water content and the problems that normally accompany high water content lenses.